The nitroxide spin probe ESR method was applied to the study of chain aggregation in aqueous solutions of poly(ethylene-co-methacrylic acid) (EMAA) ionomers. The probes selected differed in their hydrophilicity and in the position of the nitroxide group with respect to the head group. spectral sites were detected for the more hydrophilic probes and were assigned respectively to probes with restricted mobility incorporated in large aggregates and to probes with high mobility dispersed in the water phase and/or in the proximity of single-chain micelles (unimers); only the site with restricted mobility was detected in the more hydrophobic probes. The spectral parameters for the site associated with the aggregates suggest that the probes are located in different regions of the aggregates and are faithful reporters on the local hydration and polarity. On the basis of analysis of ESR spectra for six spin probes, we suggest that the aggregates present in aqueous solutions consist of three main regions : a hydrophobic core, an intermediate layer that contains both ionomer chains and some ions, and a hydrophilic region where most of the ions are located. The results for the solutions were compared with results obtained for ionomer membranes equilibrated with water. This study has revealed important structural differences between the aggregates in EMAA ionomers, and in the perfluorinated ionomers (PFI) that were studied previously by the method used in this study. The most important difference is the gradual increase in hydration of the EMAA aggregates from the hydrophobic core to the solvent-ionomer interface, compared to the complete phase separation into ionic and nonpolar domains in the PFI.